1. Field of the Invention
This invention relates to novel and unique methods for fabricating Land Grid Array (LGA) interposer contacts that are both conducting and elastic. Furthermore, the invention also pertains to LGA interposer contacts as produced by the inventive methods.
2. Discussion of the Prior Art
LGA interposers provide in general, interconnections between a multi-chip module (MCM) and a printed wiring board (PWB). A general example of an LGA retained in position between a module and a PWB is shown in the drawings, whereby LGAs may accommodate very dense two-dimensional (2-D) arrays of I/Os, and are currently being employed in the highest-end server and supercomputer products.
One widely commercially available LGA uses button contacts each consisting of siloxane rubber filled with silver particles, such as shown in FIG. 1 of the drawings. This structure is intended to provide a contact which possesses a rubber-like elasticity with the provision of an electrical conductivity. While siloxane itself has very desirable properties for this type of application, incorporating both a low elastic modulus and high elasticity, the particle-filled siloxane rubber system loses a significant proportion of these desirable properties under the loadings which are required for electrical conductivity. Although the modulus increases, it remains low overall and requires only about 30 to 80 grams per contact to ensure good electrical reliability; however, the loss of elasticity, results in severe creep deformation under constant load and stress relaxation under constant strain. These tendencies render electrically conductive elastomer LGAs unreliable for high end products which require an extraordinary stability over a lengthy period of time. Indeed, modern high end server CPUs demand LGA failure rates at ppb levels on a per contact basis because of a total system dependence on individual signal contacts.
Because of the adverse extent of creep and stress relaxation which has been demonstrated by the filled electrically-conductive elastomer LGAs of the prior art, the industry presently favors the use of LGA arrays which are fabricated from random coil springs, such as, for instance, a product called the Cinch connector which is made by the Synapse company. These springs have a much higher spring constant than the electrically-conductive elastomer type, and require pressures of upwards of 120 grams per contact in order to ensure reliable electrical connection across the array. These forces combine across a multichip module (MCM) with more than 7000 I/Os to resultingly require nearly a ton of continuously applied force over the area of a 4×4 inch module. These high forces can deform ceramic modules, reducing their planarity which can result in cracked die, broken C4 connections, and loss of thermal conductivity to the heatsink by introducing defects in normally employed thermal paste.
There is a strong technical motivating factor for using LGAs instead of rigid direct solder attachments between module and printed wiring board. The lateral stresses that occur due to TCE (thermal coefficient of expansion) mismatches between ceramic modules and organic PWBs are large, and direct ball grid array type connections often tend to fail. Systems are accordingly preferred which have some built in lateral compliance. One direct attach solution to address that problem is the so called “column grid array” or CGA. The CGA is a permanent solder type interconnect that deforms without failing in order to accommodate the lateral stresses imposed.
There is also present a strong economic motivating factor for using LGA interposers over direct attach solutions. This is because repairs and upgrades to chipsets cannot be carried out in the field with direct attach solutions. Pressure mounted LGAs can be replaced in the field, thereby saving the customer significant costs in disassembly, shipping, and rework down-time.
Thus, there are both technological and economic advantages to the pressure applied type LGA interposer approach. No current solutions provide a reliable connection that is either scalable to larger I/O counts or to more fragile organic chip module systems.
Accordingly, the present invention is directed to the provision of a device type and methods of fabrication that fulfills the combined requirements of adequate long term reliability, low contact force, and field replacablity.